<!DOCTYPE html>
<html lang="zh">
<head>
    <meta charset="UTF-8">
    <title>01-font-size_02</title>
    <style>
        body {
            font-size: 25px;
        }

        div {
            font-size: 2em; /* 2 * 25px = 50px */
        }

        span {
            font-size: 1.5em; /* 1.5 * 50px = 75px*/
        }

    </style>
</head>
<body>
我是body

<div>
    我是div
    <span>我是span</span>
</div>

</body>
</html>